Development of a gravimetric system using a conical rotating double-wing diverter for low liquid flow rates

An improved gravimetric system for low flow rates was developed at National Metrology Institute of Japan (NMIJ) to perform calibration method of flying-start-and-stop. The present paper describes the main features of the gravimetric system comprising three main components - a compact and umbrella-like conical rotating double-wing diverter, a weighing tank with a chimney-like inlet structure, and a liquid discharge mechanism using a suction nozzle - with explanation of the design considerations behind each feature. Performance tests show that, under the most unfavorable conditions, diverter timing error relative to liquid collection time is below 0.003% whereas evaporation rate of weighing tank relative to amount of liquid collection is below 0.005%. Dependency of diverter timing error on flow rates due to liquid collection loss was also revealed, and this shortcoming was improved by either one of the two following countermeasures: adjustment of nozzle sizes or modification of wing shape. These two measures are both effective in achieving a consistent timing error with varying flow rates. The overall performance results confirm the gravimetric system's capability in performing low flow calibration.